Implement general morph function for user supplied Python function

[Luiskitsu]

General morph function, MorphFuncy, that applies a user-supplied Python function to the y-coordinates of morph data (later for x-axis). This will allow the y-values to align with a target. Initially, we’ll create this general function and then we will integrate with PDFgetX3.
The UC is:

1. Luis wants to apply a user supplied python function to the y-coordinates of his morph data to make it agree with the target
2. Luis supplies the x-morph, y-morph and the function, where function is a python function that is a function of x
3. diffpy.morph applies the function to the morph and returns the morphed function

[Luiskitsu]

The MorphFuncy has been implemented. Now we need to add a MorphFuncx that applies a user-supplied Python function to the x-coordinates of the morph data.

[sbillinge]

closed by #186